The present invention relates generally to wireless communications, and more particularly, to exploiting hybrid channel information for downlink multi-user scheduling.
Consider a cellular downlink with multiple users (receivers) and a base-station transmitter, where multiple antennas are present at the transmitter. Over such a downlink, linear transmit precoding is known to be advantageous in enabling simultaneous transmissions to multiple users over the same time-frequency resource (a.k.a. multi-user multiple-input multiple output MU-MIMO).
However, the conventional approach for MU-MIMO requires accurate and timely (i.e., delay free) channel state information (CSI) from all the users in order to construct such precoders. Unfortunately, obtaining such accurate and timely CSI is near impossible in practical cellular systems and in the absence of such CSI, the conventional MU-MIMO breaks down and does not offer any gains.
The design of linear precoding schemes and user scheduling algorithms has been considered for multiple user multiple-input multiple-output MU-MIMO situations under the assumption that accurate and timely (i.e., delay free) estimates of CSI from all users can be obtained. These estimates are determined based on the CSI feedback from the users. Prediction based approaches to obtain such estimates have been proposed for the case where the delay in CSI feedback is small enough and a model for the channel evolution is available. However, at-least one of the latter two assumptions does not typically hold
A new approach suggested for MU-MIMO, referred to as the MAT scheme, has demonstrated that accurate but arbitrarily delayed CSI can also be exploited to achieve considerable MU-MIMO gains. However, the MAT scheme was proposed for a rather simple setup in which network utility maximization (NUM) via optimized scheduling was not incorporated, and thus it cannot be used over practical systems where the number of users is usually much larger than the number of streams that can be simultaneously scheduled over any time-frequency resource, which makes user scheduling necessary.
Accordingly, there is a need for exploiting hybrid channel information for downlink multi-user scheduling that overcomes the above mentioned limitations.